Photonic devices formed of high-purity molybdenum oxide

ABSTRACT

The present invention is directed to photonic devices which emit or absorb light with a wavelength shorter than that GaN photonic devices can emit or absorb. 
     The devices according to the present invention are formed using molybdenum oxide of a high purity as a light emitting region or a light absorbing region. New inexpensive photonic devices which emit light with a wavelength from blue to deep ultraviolet rays are realized. 
     The devices according to the present invention can be formed at a temperature relating low such as 700° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor photonic devices formedof high-purity molybdenum oxide which emit or absorb light with a shortwavelength.

More particularly, the present invention relates to new light emittingdiodes which emit blue light and have possibility to overcome problemsaccompanying to devices made up of known semiconductors such as galliumnitride (GaN) or silicon carbide (SiC). Moreover, the invention relatesto photonic devices which emit light with a wavelength shorter than 361nm in which GaN light-emitting diodes can emit or selectively absorblight having a wavelength shorter than 361 nm.

2. Related Background Art

Light emitting diodes which emit blue light have developed recently inorder to realize three primary colors of light and to obtain light witha shorter wavelength for digital video disc (DVD). Developed blue-lightemitting devices use gallium nitride (GaN) as an active region which isvery important to emit light. The bandgap of GaN is about 3.43 eV whichcorresponds to a wavelength of 361 nm. Although blue light can beobtained from GaN devices, there are some difficult problems. At first,bulk crystal of GaN has not been obtained because an equilibrium vaporpressure of nitrogen is very high relative to that of gallium.Therefore, substrates made up of sapphire or silicon carbide (SiC) areused. GaN cannot be formed directly on a sapphire substrate becausethere is lattice mismatch of 16% between sapphire and GaN. Therefore abuffer layer of aluminum nitride (AlN) is formed on a sapphire substratebefore growth of GaN. AlN is resistive because it is difficult to dopeimpurities into AlN. A structure and its fabrication process, therefore,are severely restricted. On the other hand, SiC substrates are veryexpensive because bulk crystal of SiC can be grown at a very hightemperature of 2200-2400° C.

Zinc oxide (ZnO) has possibility to be used to form a blue-lightemitting device. However, its bandgap is 3.2 eV which corresponds to alight wavelength of 387 nm which is larger than that GaN devices emit.Moreover, ZnO has many problems to be solved to realize practicaldevices.

The shortest wavelength of light which semiconductor photonic devicescan emit at present is that GaN devices can emit. The maximum density ofDVD memory is decided by the wavelength. Therefore, a new photonicdevice which can emit light with a shorter wavelength is expected inorder to increase the maximum density of DVD memory or to replace gaslasers such as He—Cd laser. In addition, a new blue-light emittingdevice made up of new material is expected because present blue-lightemitting devices have many problems as described above. Moreover, a newdevice which can emit light with a wavelength shorter than 361 nm whichGaN devices can emit or a shorter wavelength of deep ultraviolet rayssuch as 250-350 nm is expected.

The problem to be solved to realize a new device is to obtain a newsubstrate which replaces expensive substrate such as sapphire or SiC.

The second problem is to realize new semiconductor which can be grown ata lower temperature at which GaN or SiC layers are formed. Large energyis necessary to form semiconductor layers at a high temperature. Inaddition, there are possibilities that atoms move between layers and acomposition is disturbed or dopants move near the interface betweenlayers. It is necessary to form layers of GaN or SiC at a temperaturehigher than 1000° C.

SUMMARY OF THE INVENTION

The present invention is directed to photonic devices which emit orabsorb light with a wavelength shorter than that GaN photonice devicescan emit or absorb.

The devices according to the present invention are formed usingmolybdenum oxide of a high purity as a light emitting region or a lightabsorbing region. New inexpensive photonic devices which emit light witha wavelength from blue to deep ultraviolet rays are realized.

The devices according to the present invention can be formed at atemperature relatively low such as 700° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the optical reflection characteristics of the molybdenumoxide formed by oxidation of high-purity molybdenum at 550° C.

FIG. 2 shows the Raman scattering spectra from molybdenum oxides formedby oxidation of high-purity molybdenum at various temperatures from 450to 650° C.

FIG. 3 shows the X-ray diffraction spectra from molybdenum oxides formedby oxidation of high-purity molybdenum at various temperatures from 450to 650° C.

FIG. 4 shows temperature dependence of the electrical resistance ofmolybdenum oxide formed by oxidation of high-purity molybdenum at 550°C.

FIG. 5 is a schematic view of a structure of the light-emitting diodeaccording to an embodiment of the present invention.

FIG. 6 is a schematic view of a structure of the laser diode accordingto the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in greater detail to preferred embodiments ofthe invention.

The problems described above were resolved by using high-puritymolybdenum oxide as a light emitting region of photonic devices.

Molybdenum oxide has been studied for catalyst and its properties areshown for example in the following paper. Martin Lerch, ReinhardSchmäcker, Robert Schlögl, “In situ Resonance Raman Studies ofMolybdenum Oxide Based Selective Oxidation Catalysts” Fachbereich Chemieder Technischen Universität Berlin zur Erlongung des akademischenGrades, März 2001, Berlin.

The paper is included as a reference literature of this specification.However, application of molybdenum oxide to photonic devices, such as alight emitting diode or a laser diode is not proposed in the paper.Although the bandgap of molybdenum oxide is reported as 2.9-3.15 eV inpage 8 of the paper, any effects obtained by using molybdenum oxide inphotonic devices are not described. The values of the bandgap, 2.9-3.15eV, are the results for molybdenum oxide formed by physical method suchas sputtering or deposition in vacuum. In addition, a purity of thesample, that is molybdenum oxide, is not shown in the paper. In general,semiconductor material used in photonic devices is high-purity crystaland its bandgap is measured for such crystal. However the bandgap shownin the above paper is that of molybdenum oxide formed by deposition invacuum because molybdenum oxide is considered as catalyst in the paper.Material formed by deposition is usually amorphous and it is well knowto the peoples in the art that the material has disordered structure. Inaddition, a thickness of a film formed by deposition in vacuum isgenerally small such as 100 nm and a thickness of 1 μm is too large tobe formed by deposition in vacuum. When a thickness is small size suchas 100 nm, properties such as a bandgap of a film are affected by asubstrate and change with a thickness of a film or material of asubstrate. The bandgap shown above was obtained for such films withsmall thicknesses and was not necessarily identical to that inherent tocrystalline molybdenum oxide with a larger thickness such as 1 μm. Thereason why a bandgap was not measured for crystalline molybdenum oxidewith a thickness larger than 100 nm in the paper described above isconsidered that application of molybdenum oxide to photonic devices suchas a light emitting or laser diodes was not intended in the paper.

The inventor of this invention measured properties of the molybdenumoxide formed by oxidation of a molybdenum plate with a purity of 99.99%in oxygen atmosphere with a purity of 99.9995%. FIG. 1 shows the opticalreflection characteristics of the molybdenum oxide formed by oxidationof the molybdenum plate at 550° C. for 120 minutes. A thickness of themolybdenum oxide was 10.2 μm. The longest wavelength at which absorptionbegins, that is at which reflection is zero which is obtained byextrapolating the spectra shown in FIG. 1 gives the bandgap of themolybdenum oxide. Light with a wavelength shorter than 388 nm wasabsorbed for this sample. It means that the bandgap of the sample was3.66 eV. Because a thickness of the sample was 10.2 μm, there is noeffect of the substrate and the value of the bandgap must be oneinherent to molybdenum oxide. The reason why the value of the bandgap3.66 eV is larger than that 2.9-3.15 eV reported by Martin Lerch et alas shown in the above paper is considered as follows. It is well knownin the art that material with disordered structure such as a film formedby deposition in vacuum forms so-called band tail in the forbiddenregion of the energy band structure and its effective bandgap isdecreased. The value reported by Martin Lerch et al was obtained forsamples with disordered structure. On the other hand, the value obtainedby the inventor is that for the high-purity crystalline molybdenumoxide. Therefore the value of the bandgap measured by this inventor waslarger than those reported by Martin Lerch et al. Following data show indetail the results for high-purity crystalline molybdenum oxide obtainedby this inventor.

FIG. 2 shows the Raman scattering spectra and FIG. 3 shows the X-raydiffraction spectra from the molybdenum oxide formed similarly to thatshown in FIG. 1 except that the molybdenum oxide was obtained byoxidation at a temperature from 450 to 650° C. The spectra shown inFIGS. 2 and 3 mean that the main composition of the molybdenum oxide wasMoO₃. However it is possible that other compositions were included underthe detection limit. The bandgap obtained from the optical reflectionspectra as described for FIG. 1 was 3.45-3.85 eV for the molybdenumoxide formed at 450-650° C.

A bandgap is affected by structure, that is crystal or amorphous,disorder of crystal, a size of crystalline particle if the material ispoly-crystalline, or strain even the material has the same composition.

Therefore it should be notified that molybdenum oxide with a compositionof MoO₃ does not have always the bandgap of 3.45-3.85 eV. In otherwords, the bandgap of 3.45-3.85 eV depends on structure and strain aswell as composition. The spectra shown in FIG. 3 consist of sharp peaksand it means that the sample is pure crystal. Moreover, there ispossibility that a larger bandgap will be obtained by making quality ofthe crystal better.

FIG. 4 shows temperature dependence of electrical resistance of themolybdenum oxide whose optical reflectance property is shown in FIG. 1.As shown in the figure, resistance decreases with increase oftemperature. It means that a carrier density increases with increase oftemperature and it is phenomenon only semiconductor shows. That is,electrical conductivity which is reciprocal to resistance is determinedby a carrier density and carrier mobility. Carrier mobility decreaseswith increase of temperature because effects of lattice vibrationincrease with temperature. Therefore if a carrier density does notincreases with temperature such as metal or insulating material,conductivity decreases with increase of temperature and resistance willincrease. FIG. 4 shows as well as FIG. 1 that the molybdenum oxide issemiconductor.

As shown above, crystalline molybdenum oxide can be obtained byoxidizing a molybdenum plate at a temperature lower than 650° C. Ahigh-quality molybdenum oxide layer can be grown, for example, by vaporphase growth on a buffer layer of molybdenum oxide which has been grownpreviously on molybdenum oxide, for example, by vapor phase depositionon molybdenum oxide formed by oxidation of a molybdenum plate. Vaporphase growth of molybdenum oxide can be done at a temperature lower than650° C. by a method which will be described in the other patentapplication. Therefore light emitting devices using molybdenum oxide canbe fabricated fundamentally at a temperature lower than 650° C. using amolybdenum plate. Other materials such as aluminum (Al) crystal or Zincsulfide (ZnS) can be used as a substrate. Lattice mismatchs betweenmolybdenum oxide and aluminum and between molybdenum oxide and zincsulfide are 2.0% and 3.1%. They are much smaller than lattice mismatchbetween sapphire and gallium nitride, which is 16%. The problemsaccompanying to the present blue-light emitting devices, which are useof expensive substrates, growth at a very high temperature andcomplicated structures and fabrication process, are resolved by forminglight emitting devices using fundamentally molybdenum oxide, and lightwith a wavelength shorter than 361 nm can be obtained. In addition,molybdenum oxide is used to form devices for which a smaller bandgap ispreferable, the bandgap of the devices being controlled, for example, bydoping of impurity.

FIG. 5 shows schematically a structure of a light emitting diode (1)according to the first embodiment of the present invention. In thisembodiment, a substrate (2) is a plate of molybdenum. However othermaterial can be used as a substrate as far as it is electricallyconductive. A layer (3) consists of molybdenum oxide formed by oxidizinga surface region of the molybdenum substrate (2).

The layer (3) was formed by oxidizing a molybdenum plate with a purityof 99.99% at 550° C. in an atmosphere of oxygen with a purity of99.9995% and its thickness is 6.0 μm. Although the layer (3) is notintentionally doped, it is n type. It is considered that oxygenvacancies act as donors. A buffer layer (4) is formed on the layer (3)in order to confine disorder in the layer (3) which originates becausethe layer (3) has a different composition from the substrate (2). Forexample, the layer (4) consists of molybdenum oxide formed, for example,by vapor phase deposition at 630° C. and is n type with a carrierdensity of 3×10¹⁷ cm⁻³. It's thickness is 4.0 μm. A layer (5) ofmolybdenum oxide is formed on the layer (4). The layer (5) is formed,for example, by vapor phase deposition at 600° C. and consists ofcrystal whose quality is better than that of the layer (4). The layer(5) is n type with a carrier density of 6×10¹⁶ cm⁻³. A thickness of thelayer (5) is 3.0 μm. It is not necessary to form the layer (5) when itis not necessary to make efficiency of the light emitting diode (1) ashigh as possible. A layer (6) of p-type molybdenum oxide is fanned onthe layer (5). The layer (6) is doped, for example, with magnesium to ahole density of 1.0×10¹⁷ cm⁻³. A thickness of the layer (6) is 2.0 μmand formed for example, by vapor phase deposition. An electrode (7) isformed on the layer (6). The electrode (7) has a shape of doughnut(ring-shape) in order not to obstruct emission of light. Although theelectrode is made up of gold in this embodiment, other metals can beused for the electrode. The electrode (7) is the upper electrode of thelight emitting diode and the conductive molybdenum substrate acts as thebottom electrode. Characteristics of the light emitting diode (1)obtained by simulation are as follows. A voltage at the forward vias was10V when current was 20 mA, a light power was 60 μw when current was 20mA, and a peak wavelength was 330 nm.

FIG. 6 shows a laser diode (100) according to the second embodiment ofthe present invention. Although a substrate (101) is a molybdenum plate,other materials can be used as substrates as far as they are conductive.The substrate (101) is desirable to be conductive. A layer (102) isformed by oxidizing a surface region of the substrate and consists ofmolybdenum oxide. The layer (102) was formed by oxidizing the molybdenumsubstrate with a purity of 99.99% in an atmosphere of oxygen with apurity of 99.995% at 550° C. for 40 minutes. The layer (102) shows ntype although it is not intentionally doped. As described for the firstembodiment, it is considered that oxygen vacancies act as donors. Abuffer layer (103) is formed on the layer (102) in order to confinedisorder in the layer (102). The disorder is introduced because thelayer (102) has a different composition to the substrate (101). Thelayer (103) consists of molybdenum oxide formed by, for example, vaporphase deposition at 630° C. and is n type with a carrier density of3×10¹⁷ cm⁻³. A thickness of the layer (103) is 3.0 μm. A layer (104) ofchromium molybdenum oxide (Cr_(0.1)Mo_(0.9)O₃) is formed on the layer(103). The layer (104) of chromium molybdenum oxide has a larger bandgapthan molybdenum oxide and acts as a cladding layer which confinescarrier and light in an active layer of the laser diode. Although thelayer (104) is not intentionally doped, it is n type with a carrierdensity of 6×10¹⁶ cm⁻³. It is formed, for example, by vapor phasedeposition at 600° C. and its thickness is 3.0 μm. A layer (105) of a ptype molybdenum oxide is formed on the layer (104) as an active layer ofthe laser diode (100). The layer (105) is formed, for example, by vaporphase deposition with doping to a hole density of 1×10¹⁷ cm⁻³. Athickness of the layer (105) is 0.5 μm. A layer (106) of chromiummolybdenum oxide (Cr_(0.1)Mo_(0.9)O₃) is formed on the layer (105). Alayer (106) has a larger bandgap than the active layer (105) ofmolybdenum oxide and acts as a cladding layer of the laser diode (100).The layer (106) is formed, for example, by vapor phase deposition andhas a thickness of 3.0 μm. The layer (106) is doped, for example, withmagnesium to p type with a hole density of 4.0×10¹⁷ cm⁻³. A layer (107)of silicon dioxide is formed on the layer (106) except a central striperegion (108). Because silicon dioxide is resistive, current is limitedin the stripe region (108). The silicon dioxide layer (107) is formed,for example, by sputtering and has a thickness of 100 nm. An electrodelayer (109) is formed on the layer (107) and in the stripe region (108).Although the electrode layer (109) is formed by vacuum deposition in anembodiment, other materials and other deposition methods can be used.The layer (109) is the upper electrode of the laser diode (100) whilethe substrate (101) acts as the bottom electrode because the substrateis conductive. A width of the stripe region (108) is, 20 μm in thisembodiment. A length of the stripe region is 500 μm in this embodiment.

FIG. 6 shows one edge surface of the laser diode (100) and another edgesurface is parallel to the edge surface apart from it by a length of thestripe (108). A pair of the parallel surfaces form a Fabry-Perotresonator of the laser diode (100). Function of a Fabry-Perot resonatorin a laser diode is well known in the art. The two edge surfaces arehalf mirror in order to form a Fabry-Perot resonator. In thisembodiment, the edge surfaces were formed by reactive ion etching usingCF₄ and H₂ gases because cleavage cannot be used since the substrate(101) is molybdenum which is not crystal and hard. However other methodscan be used to form the edge surfaces.

Characteristics of the laser diode (100) were shown by simulation asfollows. A threshold current density and a threshold voltage were 5.05kA/cm² and 16.2V, respectively at pulse oscillaton of 5 μs/1 kHz. A peakwavelength was 330 nm.

FIG. 6 shows only essential elements of a laser diode and other elementscan be added to improve characteristics of the laser diode. For example,a low resistive p type layer is formed on one cladding layer (106) inorder to improve characteristics of an electrode.

Although in the embodiment shown in FIG. 6 the cladding layers (104) and(106) consist of chromium molybdenum oxide (Cr_(0.1)Mo_(0.9)O₃),chromium molybdenum oxide with other compositions (Cr_(x)Mo_(1-x)O₃,X>0.1) or other materials can be used as far as they have largerbandgaps than that of molybdenum oxide.

Details of the present invention have been described with reference tothe embodiments of a light emitting diode and a laser diode. Meritsobtained from the fact that high-purity molybdenum oxide has a largebandgap are useful in other photonic devices based on the principle ofthe present invention. Such applications of the present invention areeasily derived in the art and they are included in the scope of thepresent invention.

For example, molybdenum oxide is used in devices such asphoto-conductive devices, photo-diodes, photo-transistors, CCD and solarcells. Molybdenum oxide is used in photo-absorption regions of suchdevices.

1. A semiconductor photo-device wherein molybdenum oxide is used in atleast one layer which converts electrical energy to light or light toelectrical energy.
 2. The semiconductor photo-device according to claim1, wherein said at least one layer comprises at least a part of alight-emitting or light-absorbing region in a photo-conductive device,and said light-emitting or light-absorbing region composes at least apart of a photo-conductive device, a photo-diode, a photo-transistor, alight-emitting diode, a semiconductor laser, a solar cell or a CCD. 3.The semiconductor photo-device according to claim 1, wherein saidmolybdenum oxide has a high purity property so that efficient conversionfrom electrical energy to light or from light to electrical energyoccurs in said molybdenum oxide region.
 4. The semiconductorphoto-device according to claim 1, wherein said molybdenum oxide is ahigh purity molybdenum oxide which is formed by vapor phase depositionat a temperature lower than 700° C.
 5. The semiconductor photo-deviceaccording to claim 1, wherein said molybdenum oxide is crystallinehaving a high purity and has a bandgap of 3.45-3.85 eV.
 6. A lightemitting diode comprising: a layer of molybdenum oxide on a substrate; alayer of n-type molybdenum oxide; and a layer of p-type molybdenum oxideso that said layer of n-type molybdenum oxide and said layer of p-typemolybdenum oxide forms a pn junction from which light is emitted.
 7. Alight emitting diode comprising: a layer of molybdenum oxide on asubstrate; a buffer layer of molybdenum oxide on said molybdenum oxidelayer; a layer of n-type molybdenum oxide on said buffer layer; and alayer of p-type molybdenum oxide on said n-type layer.
 8. Laser diodescomprising a layer of molybdenum oxide on a substrate; a first claddinglayer of n-type semiconductor on said molybdenum oxide layer, said firstcladding layer having a bandgap larger than that of said molybdenumoxide; an active layer of p-type molybdenum oxide on said first claddinglayer; and a second cladding layer of p-type semiconductor on saidactive layer, said second cladding layer having a bandgap larger thanthat of said molybdenum oxide.
 9. A laser diode comprising: a layer ofmolybdenum oxide on a substrate; a buffer layer of molybdenum oxide onsaid layer; a first cladding layer of n-type semiconductor on saidbuffer layer, said first cladding layer having a bandgap larger thanthat of said molybdenum oxide; an active layer of p-type molybdenumoxide on said first cladding layer; and a second cladding layer ofp-type semiconductor on said active layer, said second cladding layerhaving a bandgap larger than that of said molybdenum oxide.
 10. Thelight emitting diode according to claim 6, wherein said substrate iscomposed of molybdenum.
 11. The laser diode according to claim 8,wherein said substrate is composed of molybdenum.
 12. The laser diodeaccording to claim 8, wherein said first and second cladding layers arecomposed of chromium molybdenum oxide.
 13. The light emitting diodeaccording to claim 7, wherein said substrate is composed of molybdenum.14. The laser diode according to claim 9, wherein said substrate iscomposed of molybdenum.
 15. The laser diode according to claim 9,wherein said first and second cladding layers are composed of chromiummolybdenum oxide.
 16. The laser diode according to claim 11, whereinsaid first and second cladding layers are composed of chromiummolybdenum oxide.
 17. The laser diode according to claim 14, whereinsaid first and second cladding layers are composed of chromiummolybdenum oxide.